The aim of the proposed research is to elucidate the roles played by cell-mediated immunity and by the antibody-forming system in oncogenesis. We have earlier established that not only cell-mediated immunity, but humoral immunity as well, is important for host defense in avian reticuloendotheliosis, a malignancy caused by an RNA tumor virus outside the avian leukosis-sarcoma complex. We propose to extend these studies into tumor prevention and cure using different cell populatations from immune and non-immune animals, and using immune and normal serum and serum components. To assess whether both cell-mediated immunity and the antibody-forming system have a host-protective function in the case of other tumors as well, the effects of thymectomy and of bursectomy for the development of other avian malignancies will be investigated. The in vivo studies will be complemented by in vitro studies into the mechanisms by which these host-protective effects are afforded.